Enameling grade steel and method of producing the same

ABSTRACT

Enameling grade steel ingots or sheets made therefrom, the ingots having a relatively thin outer rimmed layer and a thicker core, the latter being enriched by an alloying metal or metals, especially consisting of Cr, Mn, A1, V, and Ti, which are capable of preventing carbon migration toward the surface or capable of gathering the carbon precipitating in the outer layer toward the core portion during heat treatment or enamel firing above the A1 transformation point. Also disclosed is a method of making the ingots.

States Patent Unite [21] Appl. No. [22] Filed [45] Patented [73]Assignee [54] I ENAMELIING GRADE STEEL AND METHOD OF PRODUClNG THE SAME3 Claims, 15 Drawing Figs.

[52] US. Cl 29/196.1, 75/126,148/34,148/39,164/57,164/1l7 [51] Int. Cl13326 15/00 [50] Field of Search 75/123, 126, 129,130; 148/12, 39, 12.1,34, 39; 164/55, 56,57;29/186.5,187.5,196.1

[56] References Cited UNITED STATES PATENTS 2,108,254 2/1938 Devaney75/58 X 2,978,765 4/1961 Brown 164/57 X 3,219,438 11/1965 Poole 164/55 X3,392,063 7/1968 Kohlen. 148/12 2,236,504 4/1941 Herty 75/45 2,356,4508/1944 Epstein. 75/123 2,736,648 2/1956 Eckel 75/123 OTHER REFERENCES l.S. Epstein, et al., New Vanadium Steel for Deep Drawing Sheets,ln TheIron Age. p. 158- 163. Oct. 12, 1950 Primary Examiner-L. DewayneRutledge Assistant Examiner-Joseph E. Legru Attorney-McGlew and TorenABSTRACT: Enameling grade steel ingots or sheets made therefrom, theingots having a relatively thin outer rimmed layer and a thicker core,the latter being enriched by an alloying metal or metals, especiallyconsisting of Cr, Mn, Al, V, and Ti, which are capable of preventingcarbon migration toward the surface or capable of gathering the carbonprecipitating in the outer layer toward the core portion during heattreatment or enamel firing abovethe A, transformation point. Alsodisclosed is a method ofmaking the ingots.

PATENTEUUBT 26 197i FIG. 1

SHEET NF 6 ANALYSIS OF SLAB OF 140'' THICKNESS o I b", 0 0

T surface central sunlace surface cenrml surface surface central surfaceP" SLAB THICKNESS |---SLAB THICKNESS at 50" height of ingot P-SLABTHICKNE$S-- m at [8 height of ingot from bottom PATENTEUum 26 mm SHEET3UF 6 t Lu 0.20- 5 Lu 8 E g (1/0- surface central surfm f'* SHEETTHICKNESSH 0 surface central surface PATENTEDBU 2s :97:

CHROHIUF f IN PERCENT SHEET MF 6 surface Cenfrfll surface W- SHEETTHICKNESS 0 ATTOF/VE Y5 PATENTEUnm 2s |97| 361 51.278 SHEET 8 0F 6 FIGO7 ENAMELIING GRADE STEEL AND METHOD OF PRODUCING THE SAMECROSS-REFERENCE TO RELATED APPLlCATlON This is a continuation-in-partapplication of Ser. No. 41 7,009, filed Dec. 9, 1964, now abandoned.

SUMMARY OF THE INVENTION The present invention relates to enamelinggrade steel ingots and sheets made therefrom. The ingots have arelatively thin outer rimmed portion and a thicker core, the latterhaving incorporated therein predetermined amounts of an alloying metalor metals, which prevent the migration of carbon toward the surface orwhich are capable of gathering the carbon precipitating in the outerlayers toward the core portion during heat treatment or enamel firingabove the A transformation point. The invention also relates to a methodof producing such steel ingots and steel sheets.

The products made according to the invention exhibit superior propertiesfor enameling purposes and have very satisfactory cold workability. Anumber of other advantages of the products will be discussed in furtherdetail in the course of the following description.

Prior art low-carbon steel sheets have a great tendency to bend duringenameling operations and small projections or crater-like pinholesappear on the enameled surface, thus detrimentally affecting theappearance and quality. These surface defects are usually referred to asblister. In the surface layer immediately below these blisters, thepresence of gathering carbides can be observed. These blisters arecaused by escaping CO or CG! gas fonned in the reaction between oxygenin the enameling chemicals and carbon in the steel.

in a steel high in carbon content, a surface defect called fish scale"often appears on the enameled surface, which is caused by large amountsof hydrogen escaping from steel during or after the enameling firingprocess.

As mentioned above, these surface defects are caused by the carboncontained in the steel. Efforts have been made to eliminate thesedefects. For this purpose, improved enameling grade steels havepreviously been developed, for example, and extremely low-carbon steelin which the carbon content is maintained below 0.03 percent is producedby a special steel making process or by deearburization annealing.Further, titanium containing steel has been suggested in which thedetrimental effects of carbon are alleviated through carbide formationof the added titanium with the carbon.

The above-mentioned improved enameling grade steels have generally goodenameling properties, but in lowering the carbon content during thesteel making process the oxygen content increases in inverse proportionto the lowering carbon content, thus producing a considerable amount ofthe nonmetallic inclusion in the produced steel. Further, the coldworkability of the produced steel sheets is lowered in this manner.

Conventional enameling grade steel, in which carbon is chemically boundby addition of an element such as titanium to form carbides with thecarbon, can only be produced in killed form, which results in pooryields. This is so in order to attain alloying conditions in appropriateproportion to the carbon content and oxygen content in the alloyingelements and their cold-forming properties are lowered. Moreover, theseconventional enameling grade steels are expensive to make. Accordingly,in practice, the ordinary low-carbon rimmed steels are mostly used forgeneral enamelwares.

Further, when the deearburization is performed in an annealing furnaceof the open coil type, the commercial application is limited to steelhaving less than about 2 mm. thickness, because the deearburization isdifficult to perform when the steel is thicker.

Accordingly, it is a primary object of this invention to produce animproved kind of steel suitable for enameling purposes.

Another objeet is to provide improved steel sheets which can be readilyenameled to exhibit superior surface characteristics without having thesurface defects referred to.

it is also an object of this invention to provide a process forproducing the improved steel and steel sheets.

Generally, it it an object of this invention to improve on the art ofenameling grade steel and methods for its production.

The inventors have thoroughly investigated the behavior of the carbidesjust below the steel surface which cause the surface defects during theenameling process of low-carbon steel sheets and have discovered thatthe surface layer (rimmed layer) of low-carbon steel ingots or hotrolled product obtained therefrom has a lower content of carbon,nitrogen, phosphorus and sulfur than the core portion. This is due tothe rimming action during the ingot making. However, when the hot-rolledsteel products or the cold-rolled products obtained therefrom areannealed and subjected to enameling firing, the carbon in the steeldiffuses and migrates during the annealing and the enameling firingprocess in a striking degree toward and into the rimmed layer in thesurface portion of the steel, as shown in FIG. 3 (b) explained below.Further, it has been discovered that this migration or gatheringtendency of the carbon is more pronounced at higher enameling firingtern peratures, particularly when the temperature is above the Atransformation point.

As explained above, in lowcarbon rimmed steel, various surface defectsoccur when the annealing is effected above the A, transformation pointbecause the carbon migrates toward the surface and gathers inconsiderable amounts in the surface layer of the steel. if the annealingis effected below the A transformation point, the migration to andgathering in the surface layer of the carbon is somewhat diminished, butcomplete prevention of the surface defects on the enameled surfacecannot be obtained thereby. The drawability of the steel is alsolowered.

Based upon the above realization and concept, the enameling grade steelof the present invention contains in its center or core portionpredetermined amounts of specific elements effective for preventingdiffusion or migration movement of the carbon toward the surface, sothat the diffusion movement of the carbon to the surface of the steelsheet during the annealing and enameling firing is prevented. Onlyincidental amounts of these elements are contained in the rimmed surfacelayer.

In the present invention, an opposite effect is in fact obtained becausethe carbon is forcibly diffused or caused to migrate toward the coreportion during firing and to gather there so that the carbon content inthe surface layer of the steel sheet is lower than in the core portion.From a practical point of view, the carbon content in the rimmed surfacelayer should preferably be lower that about 0.02 percent, whereas theabove core will contain 0.04 to 0.15 percent of carbon, whereby theenameling grade steel sheet. having excellent sur face qualities,superior to those of extremely low-carbon enameling grade steel sheets,is obtained.

The results of many experiments indicate that specific alloying metalswhich are positively effective in causing migration and gathering of thecarbon content are chromium, manganese, vanadium, titanium and aluminum.

Since these metals lower that austenite-to-ferrite transformationtemperature of the steel, the portion of the steel containingappreciable amounts of these elements, i.e., the core portion, has agreater tendency to transform into austenite than the portion containinglesser amounts of such elements, i.e., the rimmed surface layer, duringthe annealing or during the enameling firing. The steel portion whichhas transformed to austenite is capable of dissolving much largeramounts of carbon. it is believed that the core portion containingappreciable quantities of such elements has a strong power to attractand collect the carbon content in the steel, for such portion transformsto ferrite only after the remaining portions of the steel havetransformed from austenite to ferrite. Accordingly, in steel sheetswhich contain appreciable amounts of one or more of the enumeratedelements mainly in the inner or core portion, the carbon will migrateand gather in the inner portion.

Vanadium, titanium and aluminum, though not effective directly to lowerthe transformation temperature, are effective to lower the activity ofcarbon, and thus in the core portion where these elements segregate andaccumulate, the content of carbon increases and segregates, whereby thetransformation point, in turn, is lowered. Thus, these elements havesimilar effects as chromium and manganese.

The prior art teachings relating to conventional enameling 'grade steeldo not disclose the incorporation of the alloying metals in theinventive manner, i.e., so that the major portion of these metals isconfined to the core portion.

In, for example, producing enameling grade steel sheets containing 0.03to 0.10 percent of carbon in accordance with this invention, thedifference between the content of the inventive alloying metals in thecenter or core portion and the content of these metals in the surfacelayer of the steel sheet should be more that about 0.5 percent forchromium and manganese, more than 0.2 percent for titanium and vanadiumand more than 0.003 percent for acid soluble aluminum, so as effectivelyto collect the carbon content predominantly in the center portion of thesteel. However, too high a content of these elements tends to harden thesteel and to lower the cold workability of the steel. It is, therefore,desirable to maintain the content of these alloying elements as low aspossible while still obtaining the desired gathering effect. Experimentshave demonstrated that the most desirable range for the content of theseelements in the core portion, considering the desired enamelingproperties and cold workability of the steel, is 0.10 to 0.20 percentfor chromium and manganese, 0.05 to 0.15 percent for titanium andvanadium, and 0.005 to 0.002 percent acid soluble aluminum.

In accordance with this invention, the surface defects which are apt tooccur during the enameling operation of prior art steels are avoided bypreventing the diffusion of carbon to the surface.

Conventional enameling grade steel sheets, in which titanium, niobium,and the like are added, are made from killed steel, and essentially nosegregation of carbon in the interior of the sheet takes place.Moreover, titanium, niobium and the like are uniformly distributed inthe steel and thus the carbon in steel is diffused uniformly throughoutthe steel sheet during the annealing, normalizing or enameling firingoperation. Accordingly, in the conventional enameling grade steelsheets, the carbon content in the surface layer is equivalent to theaverage carbon content of the steel. 7

As will be clearly understood from the above, according to the resentinvention, it is possible to produce highly satisfactory enameling gradesteel sheets with small amounts of the specific elements, which arecontained predominantly in the center portion of the steel sheet.

In the steel sheets according to the present invention, a desirablethickness of the surface layer (rimmed layer) is more than about 0.05mm. This means that the major amount of chromium, manganese, vanadium,titanium and/or acid soluble aluminum is to be present in an inner orcore portion which terminates at least 0.05 mm. from the surface. If thesurface layer is too thin, it may be destroyed or negatively affected byoxidation during the annealing and normalization prior to the enamelingfiring proper, or during acid pickling.

Enameling grade steel sheets produced from low-carbon steel inaccordance with the invention with an enriched carbon-attracting centralor core portion have the following advantages:

1. Fewer defects on the enameled surface.

2. Higher annealing temperatures can be applied to cold rolled products;hence, better cold workability.

3. In the conventional enameling grade steel sheets, in which Ti, Nb,Zr, or Sb is added such elements are uniformly distributed throughoutthe steel, and the required amount of such elements to be added isconsiderably larger, which hardens the steel, lowers the cold formingproperties and raises the production cost. According to the presentinvention, however, the specified elements are added only to the centerportion of the steel, and thus the required amount of the elements perunit weight of the steel is much less; hence, improved cold-formingproperties and advantages in relation to the production cost.

4. According to the present invention even for a relatively thick sheetabove 2.0 mm. an extremely low-carbon surface layer can be obtained, forthe carbon content in the steel can be readily collected in the centerportion by annealing or normalizing operations.

5. When Ti, Aland V are employed as addition elements the transformationpoint of the steel from alpha to gamma rises, and thus it is possible toraise the enameling firing temperature as well as to alleviate thepossible strain encountered during the enameling firing process.

In most cases, enameling grade sheet is used in a drawn form. It isrecognized technical problem in the drawing operation that ordinarylow-carbon rimmed steel sheets or extremely low-carbon rimmed steelsheets annealed in an open coil system, are susceptible to stretcherstrain which is a critical factor for enameling grade steel sheet.

Commonly applied measures for preventing the stretcher strain which isencountered during the drawing operation of enameling grade steel sheetsare to increase the reduction rate of skin-pass rolling to applylevelling prior to drawing incase of rimmed steel sheets. However, suchmechanical treatments, such as by skin-pass rolling or levelling, tendsomewhat to lower the cold formability of the steel, require more stepsof operation, and cause aging of the steel.

The inventive enrichment of the core portion of the sheets with thespecified alloying metals effectively overcomes the stretcher stainproblem during drawing.

The present invention also relates to a method for casting steel ingotsto produce the novel steel composition.

After pouring molten steel into an ingot mold, rimming action ismaintained for several minutes, whereby a solidified peripheral layerportion is formed. Thereafter, one or several of the specified elements,to wit, manganese, chromium, aluminum, titanium or vanadium in the formof bars, lumps or powder, or alloys of these metals are introduced intothe unsolidified portion of the ingot so as to deoxidize and kill theinner portion of the molten steel, as well as to disperse these elementsuniformly through the whole volume of unsolidified molten steel. Adeoxidizer may also be added. The timing and the method of additions aswell as the form of the alloyin metals are factors to be considered.

The following is an explanation in relation to the production of aningot having a thickness of 700 to 800 mm. in which the above metals oralloys were added in the form of bars.

When the addition element or alloy is added at too early a stage, nosatisfactory rimmed layer is formed; hence, satisfactory surfacequalities cannot be expected in the steel sheets. On the other hand,when the addition is made at too late a state, the added alloyingelements will be readily distributed uniformly within the core portion.Generally, it is considered that the optimum time of adding the additionelement or ele ments is about 3 to 10 minutes after the pouring, i.e.,when the thickness of the solidified outer wall (rimmed layer) hasattained about 30 to 70 mm. The ratio of the thickness of the outersurface layer to the thickness of the core should thus preferably be 3:70-lz20,

For example, when steel sheets are produced from an ingot of 800 mm.thickness and having a rimmed surface of 40 mm. thickness, sheets of 2mm. thickness will have a 0.! mm. thick rimmed surface, and sheets of 1mm. thickness will have a 0.05 mm. thick rimmed surface.

As for the from of the addition elements or alloys, long bars,

for example, are preferable in view of the need of distributing theaddition elements uniformly in the core portion of the ingot. However,the addition elements or alloys may be incorporated in the form ofpowder or lumps accommodated in a steel pipe of below 1 mm. wallthickness, which pipe is inserted into the molten core.

In case the addition elements or alloys are introduced in the form ofbars, it is obvious that small diameter bars will necessitate a numberof such bars to be inserted. On the other hand, large diameter barsresult in a greater heat capacity, which causes solidification of aquantity of steel around the bars, and thus the melting of the additionelements or alloys will not take place speedily, uniformly, orcontinuously, or the bars will often fusion out. Accordingly, thediameter if the bars should be chosen depending inthe heat capacity andmelting point of the addition elements. However, a diameter of to 30 mm.is generally satisfactory. As for the length of the bars, the use ofvery short length bars may cause segregation of the addition elements inthe upper portion of the ingot only with poor distribution thereofwithin the core portion. It is thus preferable to employ bars having alength of about 40 percent of the ingot height. Generally, it isimportant that the placement of the bar in the molten mass of the ingotmold should be effected so as to ensure uniform distribution of theaddition elements throughout the unsolidified mass.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings in which there areillustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representationof an ingot mold during the addition of flti bars for enriching the coreportion of steel in accordance with the invention;

FIGS. 2a-2c show the distribution of soluble aluminum in slabs for coldrolled sheets produced according to the present invention, for ordinarylow-carbon rimmed steel sheets and for A l -lrilled deep drawing steelsheets, respectively;

FIGS. L la-3b show, respectively, the microstructures of the cold-rolledenameling grade sheets produced according to the present invention andof cold'rolled rimmed steel sheets produced by the conventional method,both annealed at 740 C. for three hours, heated for 4 minutes at 830 C.,almost equal to the enameling firing temperature, and quenched in saltwater;

FIGS. lla-db show, respectively, the manganese distribution incold-rolled enameling grade sheets produced according to the presentinvention using manganese as alloying component, and in rimmed steelsheets produced by conventional methods, while FIG. 4c shows themicrostructure of the coldrolled sheets of FIG. 4a as annealed at 720 C.for 6 hours, heated for 4 minutes at 830 C., almost equal to theenameling firing temperature, and quenched in salt water;

FIG. 5a shows the chromium distribution through the thickness ofcold-rolled sheets produced according to the present invention usingchromium as alloying component;

FIG. 5b shows the microstructure of the same sheet as annealed at 740 C.for 3 hours, heated at 830 C. for four minutes, and quenched in saltwater;

FIG. 6 is the microstructure of enameling grade cold-rolled sheetsproduced according to the present invention using aluminum and chromiumin combination as alloying metals, as annealed at 720 C. for 3 hours,heated for 3 minutes at 850 C., almost equal to the enameling firingtemperature, and quenched in salt water;

FIG. 7a is a photograph showing the surface appearance of conventionallow-carbon rimmed steel sheets on which cnameling glaze has been twiceapplied;

FIG. 7b is a similar photograph as indicated in FIG. 7a showing thesurface appearance of a sheet of the present invention to the centralportion of which aluminum has been added;

FIGS. 70 and 7d are, respectively, photographs showing the surfaceappearance of a conventional low-carbon rimmed steel sheet and a sheetof the present invention to the central portion of which chromium hasbeen added, both sheets hav ing been twice glazed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. I, referencenumeral l represents an ingot mold, 2 is the solidified wall orperipheral surface portion of the steel, 3 is the unsolidified steelmass, and 4 represents several bars of addition elements of relativelysmall diameter. Since the molten mass in the mold always solidifiesfrom, the periphery towards the center, the outer portion 2 solidifieswhile the core portion is still liquid, so that the bars 4 may beinserted thereinto.

FIG. 2a shows the distribution of acid soluble aluminum in a slab forthe cold-rolled sheet produced according to the present invention, inwhich aluminum in the form of a bar of 26 mm. diameter and 1,400 mm.length has been used. In FIG. 2b, there is represented for comparisonpurposes the distribution of acid soluble aluminum in a low-carbonrimmed steel while the distribution in Al-killed steel for deep drawingproduced according to conventional ingot making methods is shown in FIG.20. It is clearly seen from the graphs that the rimmed layer of thesteel sheet according to the present invention contains almost no acidsoluble aluminum, the acid soluble aluminum being concentrated uniformlyin the core portion.

FIG. 4a represents the distribution of manganese in a steel sheetproduced according to the present invention in which Fe-Mn alloygranules in a steel pipe of 0.6 mm. wall thickness, 20 mm. diameter, and1,500 mm. length have been inserted into the core portion. FIG. 4bshows, for comparison purposes, the manganese distribution in low-carbonrimmed steel produced according to conventional ingot making methods. Inthe sheet of the present invention, as compared with the prior artsheet, manganese is concentrated in the central portion rather than inthe surface layer.

The steel of the present invention may be produced in a converter, anopen hearth furnace, an electric furnace or any other suitablesteel-producing equipment. However, the content of unavoidable elementsin the molten steel as teemed, such as Si, P, S, Cu, Ni, Cr, Ti, Nb, Vetc. should preferably be maintained as low as possible, although theseelements are permitted to be present in the steel in a similar amount asin the ordinary low-carbon rimmed cold-rolled sheets. Thus, it isdesirable that the silicon content is not more than 0.0l percent, thecontents of phosphorus, sulfur, chromium and nickel are respectively nohigher than 0.030 percent, the copper content is not more than 0.10percent and the contents of niobium, vanadium and titanium arerespectively not more than 0.001 percent. Accordingly, the contents ofthese elements in the surface layer (rimmed layer) of the steel sheet ofthe present invention are almost equal or somewhat lower than those inthe molten steel as teemed, and the contents of these metals do notsubstantially influence the effects on carbon of the specified alloyingelements contained in the central portion of the steel sheets accordingto the present invention.

When the carbon content in the ladle steel analysis is too low, theoxygen content in the steel remarkably increases, thus increasingnonmetallic inclusions and weakening the rimming action so that asatisfactory rimmed layer is hard to obtain. High carbon contentrequires larger amounts of the specified alloying elements to be addedto the central portion, which, in turn, lowers the cold formability ofthe sheet. Generally, it is desirable that the carbon content is 0.04 to0. l 5 percent.

As for the manganese content in the ladle steel analysis, generally acontent of 0.25 to 0.60 percent is desirable in order to prevent redshortness due to FeS. However, for an enameling grade steel, it isdesirable that the manganese content is no more than 0.30 percent, for ahigher manganese content will increase the strain during the enamelingfiring process. In this case, the sulfur content should preferably be nomore than 0.02 percent.

The hotand cold-rolling operation of the ingot produced according to thepresent invention does not affect the essence of the present invention,and can be carried out as easily as in ordinary steel sheet production.I

By annealing or normalizing after the hot or cold rolling, carbon in thesteel migrates to and gathers in the central portion of the sheetwithout diffusing and moving to the surface layer so that a surfacelayer low in carbon content is obtained.

To enhance the activity of the alloying elements thus concentrated inthe central portion of the sheet, it is desirable that the annealing ornormalizing is effected above 720 C. and the holding time is longer.

Steel products produced from ingots in which the specified alloyingmetals have been added to the core portion after a rimmed layer ofsuitable thickness has been solidified have the following advantageouscharacteristics:

1. they have rimmed layers substantially free from surface defects;

2.surface defects do not appear during the enameling firing; and

3. hot and cold workability are good.

The invention will now be described by the following examples, it beingunderstood, however, that these examples are given by way ofillustration and not by way of limitation and that many changes may beeffected without departing from the scope and spirit of this inventionas recited in the appended claims.

EXAMPLE I An ingot was prepared from top-blown oxygen converter steel ina mold as shown in FIG. I under the following conditions:

Ladle steel analysis C: 0.07 Mn: 0.28 Si: 0.01 P: 0.009 S: 0.014

A1 added for deoxidation in ladle: 60 g./t.

Pouring temperature: I ,6 1 C.

lngot weight: l4,400 kg.

Cross section of ingot: 1,400 mm. wide and 800 mm. thick lngot height:1,800 mm.

Al bars inserted number: 4 bars size: 26 mm. diameter and 1,400 mm.length weight: 7.2 kg. (500 g./t.)

Timing of Al bar insertion: 3 minutes after the pouring.

FIG. I shows schematically the position of the inserted Al bars. Moltensteel in the ingot mold (I) solidified with rimming action for threeminutes and a solidified surface wall (2) of 35 to 40 mm. thickness wasformed. The Al bars (4) were then inserted into the unsolidified moltensteel portion (3) to a depth of 1,300 mm. from the upper surface of theingot.

Analysis of cold-rolled sheets of 0.8 mm. thickness produced from theabove ingot is as follows:

C: 0.06 percent, Mn: 0.28 percent, Si: 0.01 percent, P: 0.009 percent,

S: 0.015 percent, total N: 0.002] percent, soluble Al: 0.00l percent inthe rimmed layer, 0.008 percent in the core por' tion.

The mechanical properties of the sheets as annealed at 740 C. for threehours are as follows:

Yield point: l9.2 kg./mm.2,Tensile strength: 33.3 kg./mm.2

Elongation: 46.2 percent, Erichsen value: I 1.1 mm.,

Hardness: I-IRB 46.2.

For comparison with the product of example I, rimmed steel was producedfrom the same charge as stated in example I by a conventional ingotmaking method, i.e. the ingot was prepared in such a way that 800 g. (55g./t.) of aluminum granules were added uniformly during the pouring forthe purpose of controlling the deoxidation. Rimming action was effectedfor 21 minutes after the pouring. The ingot mold was covered with asmall lid (700 kg.).

Analysis and mechanical properties of cold-rolled sheets of 0.8 mm.thickness from the comparison ingot (rimmed steel) are as follows:

C: 0.04percent, Mn: 0.27percent, Si: 0.0lpercent, P: 0.01 lpercent,

S: 0.01 7percent, total N: 0.0023percent,

soluble AI: 0.00lpercent in the rimmed layer, and

0.001percent in the core layer. Yield point: 23.1 kg./mm.2

Tensile strength: 33.0 kg./mm.2

Elongation: 45.5percent, Erichsen value: 10.9 mm.

Hardness: I-IRB 48.5

The above two cold rolled sheets were annealed for three hours at 740C., heated for four minutes at 830 C., which is almost equal to theenameling firing temperature, and then quenched in salt water. Themicrostructures of the sheets are shown in FIGS. 3 (a) and (b).

FIG. 3 (b) is the microstructure of the rimmed sheet produced from thecomparison ingot. It can be clearly seen that the rimmed layer has aquenched structure of high carbon steel and the central portion (coreportion) has a ferrite structure with most of the carbon content beingconcentrated in the surface layer. By contrast, in FIG. 3 (a) whichshows the microstructure of the sheet produced according to the presentinvention, it is clearly seen that no high carbon phase is present inthe surface layer of the sheet, though present in the central portionwhere much of soluble aluminum is present. The surface layer is thus aferrite phase, very low in carbon content (presumably less than 0.025percent of carbon), and the diffusion movement to the surface layer ofthe carbon content is completely prevented.

Enameling tests were conducted on the above enameling grade steel sheetof the present invention and on the ordinary prior art rimmed steelsheet from the same charge under the following conditions:

Application of enameling glaze twice titan white (trade name) ofJapanFerro K.K.

Surface glaze The photographs of the surface appearance of the twoenameled sheets are shown in FIGS. 7 (a) and (b), respectively. Blistersare visible on the conventional rimmed sheet, but absent on the sheet ofthe present invention. Parts for gas cooking-pots for rice and gasranges were made from the sheet of the present invention and theconventional rimmed sheet and subjected to tests for determining theirpractical usability as enamelwares. The results were that no blisterswere detected in any of 200 parts produced from the sheet of the presentinvention, whereas in case of the conventional rimmed sheet the blisteroccurrence ratio was 10.5 percent.

Another advantageous feature of the sheet of the present invention isthe easiness with which it can be subjected to acid pickling operations.The acid-pickling speed for the sheet of the present invention is abouttwice as fast as that for the conventional rimmed steel sheet orextremely low-carbon enameling grade steel sheets decarburized by theopen coil treatment. This, of course, increases the productivity.

Further, measurements of nonmetallic inclusions in the sheets of thepresent invention corresponding to all areas of the inner portion of theingot were carried out for determining purity. A high degree of puritywas established.

Conventionally, the addition of deoxidation agents to the ingot moldwould impede the floating up or rising of deoxidation products and thusgreatly decrease the purity of the products. For this reason, thisaddition method for deoxidation agents has not been applied in practicein spite of its advantages based on deoxidation in the ingot mold.Surprisingly, the results of the present invention show that the purityof the products of the present invention is strikingly improved ascompared with that of sheets produced from ordinary rimmed steel. Theamount of nonmetallic inclusions in the products of the presentinvention is about half of that in sheets produced from the ordinaryrimmed steel. This can be attributed to the following: When aluminumbars or the like alloying metals are inserted into the molten steelrimming in the ingot mold, the aluminum or the like alloying metal meltsand a gradual killing effect takes place from within the centralportion. While this effect takes place, the convection of molten steelsets in following the rimming action for a considerable time, duringwhich a large amount of scum is seen to float up to the molten steelsurface. From this it can be inferred that the oxidation products formedin the inner portion of the molten steel float to the molten steelsurface by the action of convection and are absorbed by the scum whichhas been formed there.

EXAMPLE 2 Topblown oxygen converter steel of the following analysis waspoured into two ingot molds of the same size as in example l.

Ladle analysis: C: 0.07%, Si: 0.01%, Mn: 0.26%,

P: 0.012%, 8: 0.015%, N: 0.00l6% Pouring temperature: 16 C.

Ladle deoitidation: Al addition: 80 g./t.

After the molten steel .in the ingot molds solidified with rimmingaction for four minutes forming a solidified surface layer, alloygranules of Fe-Ti containing 45percent of titanium and Fe-V containing50 percent of vanadium, each packed in a steel pipe of 23 mm. diameter,1,500 mm. length and 0.6 mm. wall thickness, were respectively insertedinto the un solidified portion of the molten metal in the molds in asimilar way as in example 1, whereby Ti and V containing steel ingotswere respectively obtained.

Analysis of two hot rolled sheets of 3.2 mm. thickness produced fromthese ingots is as follows;

C Mn P S N Ti-containing sheet 0.050 0.27 0.016 0.018 0.0026 0.026 0230.010 0.010 0.0018 V-containing sheet 0.045 0.28 0.018 0.019 0.00270.026 0.20 0.012 0.009 0.009

Ti V

Ti-contuining sheet trace in surface trace 0.18-0.010 in central portionV-contuining sheet trace trace in surface 0.03 0.025 in central portionThese hot-rolled sheets were cold rolled to 0.8 mm. thickness andannealed at 720 C. for 6 hours. Measurements of mechanical properties ofthe annealed sheets corresponding to the central portion of ingots areshown below:

As can be seen from the above table, better Ericksen value, C.C.V.value, and elongation value are attained in the Ti-containing andV-containing sheets of the present invention than in the rimmed steelsheet produced from the same melt and heat treated under the sameconditions, though the tensile strength value is almost the same. Thisindicates that the sheets of the present invention have better pressworkability.

Enameling tests were conducted on the above sheets of the presentinvention and on the rimmed steel sheet under the same conditions as inexample 1. The results were that blisters were detected in the rimmedsteel sheet, but were absent in the sheets of the present invention. Theresults of tests for determining the usability as enamelwares conductedon 200 parts for gas cookingpots for rice produced from the sheets ofthe present invention showed that no blister formation took place in anyof the parts.

EXAMPLE 3 Topblown oxygen converter steel of the following analysis wasprepared:

Ladle analysis: C: 0.06%, Si: 0.01%, Mn: 0.22%, P: 0.008%, S: 0.01 1%,N: 0.001%

Pouring temperature: 1600 C.

Ladle deoxidation: Al addition: g./t.

This molten steel was poured into an ingot mold under the followingconditions:

lngot weight: 14,400 kg.

Cross section of ingot: 1,400 mm. width and 800 mm. thickness Ingotheight: 1,800 mm.

After the molten steel in the ingot mold solidified with rimming actionfor seven minutes and a solidified outer wall of about 60 mm. thicknesswas formed, alloy granules of Fe-Mn containing 78% of Mn packed in asteel pipe of 20 mm. diameter, 1,500 mm. length and 0.6 mm. wallthickness were inserted into the unsoliditied portion of the moltensteel in a similar position and manner as the Al bars shown in FIG. 11.

Analysis of cold rolled sheets of 0.8 mm. thickness from the above ingotis as follows:

C: 0.06%, Si: 0.01%, Mn: 0.21% in the rimmed layer,

0.32% in the core portion, P: 0.009%, S: 0.013%,

soluble Al: 0.001%.

Distribution of manganese through the thickness of the sheet of thepresent invention and of ordinary rimmed steel sheets is shown in H6. 4(a) and HO. 4 (b), respectively.

Mechanical properties of the rolled sheet of the present invention asannealed at 720 C. for 6 hours are as follows:

Yield point: 19.3 kg./mm., Tensile strength: 32.9 kg./mm.*,

Elongation in 500 mm.: 45.2%, Erichsen value: 10.7 mm.,

Hardness: HRH 44.2.

FIG. 4 (c) is a photograph showing the microstructure of the annealedsheet which was heated for 4 minutes at 830 C.. which is almost equal tothe enameling firing temperature, and quenched in salt water. The blackportion in the photograph represents the quenched structure of the phasewhich was austenite at 830 C., and from the Fe-C constitutional diagramit is calculated that this quenched structure contains about 0.2% ofcarbon. The photograph indicates that the surface layer remained asferrite even at 840 C. and the carbon content in this portion is belowabout 0.02 percent, which is the solution limit of carbon in ferrite.

Further, it was confirmed that in the structure of the above annealedsheet, even after being heated at 830 C. and air cooled, there was nosegregation of carbon in the surface layer, and the carbon segregationwas observed only in the central portion.

Enameling testing was conducted on the above sheet and on a rimmed steelsheet produced from the same charge under the following conditions:

Application of enameling glaze: twice Amount of base glaze: 3.7 gJdm.

Firing period: 10 minutes Firing temperature: 870 C.

Surface glaze: titanium white (trade name) of Japan Ferro Co., Ltd.

The results were that blisters were detected in the conventional rimmedsteel sheets whereas no blisters were seen in the sheet of the presentinvention.

The results of tests for determining practical usability of gascooking-pots and gas ranges produced from the enameled sheet of thepresent invention showed that no blister formation occurred in any ofthe 200 test pieces.

EXAMPLE 4 The molten steel of the same ladle as described in example 1was poured into an ingot mold of the same size as in example 1. Themolten steel was allowed to stand for eight minutes, whereafter asolidified surface wall of about 60 mm. thickness was formed. Alloygranules of Fe-Cr containing 65percent of chromium packed in a steelpipe of 20 mm. diameter, 1,500 mm. length, and 0.6 mm. wall thicknesswere then inserted into the molten metal in a similar manner as inexample 1.

Analysis of hot rolled sheets of 2.3 mm. thickness produced from theabove ingot is as follows:

C: 0.06%, Si: 0.01%, Mn: 0.021%, P: 0.010%,

S: 0.013%, Cr: 0.01 1% in the surface layer,

0.072% in the inner layer.

Distribution of chromium throughout the thickness of the above coldrolled sheet is shown in FIG. (a).

Mechanical properties of cold-rolled sheets of 1.0 mm. thickness,produced from the above hot-rolled sheets, as annealed at 740 C. for 3hours, are as follows:

Yield point: 21.3 kg./mm. Tensile strength: 33.0 kg./mm.

Elongation: 43.7%, Ericksen value: 10.2 mm.

Hardness: HRB 45.1.

The microstructure of the above annealed sheet as heated at 630 C. for 4minutes and quenched in salt water in a similar way as in example 1 isshown in FIG. 5 (b).

The sheet of the present invention and a conventional rimmed steel sheetproduced from the same charge were subjected to enameling tests underthe following conditions:

Glaze applied: H-type glaze of Japan Ferro Co., Ltd.

Amount applied: 3.7 g./dm.

Firing temperature: 870 C.

Firing period: ten minutes The results were that many blisters wherevisible in the rimmed steel sheet as shown in FIG. 7 (c), whereas noblister formation was observed in the sheet of the present invention, asshown in FIG. 7 (d).

Tests for determining practical usability were conducted on 500 partsfor gas cooking-pots for rice and gas ranges, produced from the sheetsof the present invention. These tests indicated that no blistersoccurred in any of these parts, whereas tests showed that blisteroccurrence in the parts produced from the conventional rimmed steelsheet was 13.7 percent.

EXAMPLE 5 Molten steel containing 0.08% of carbon, 0.01% of silicon,0.25% of manganese, 0.012% of phosphorus, 0.012% of sulfur (ladleanalysis) was poured into an ingot mold of 14.4 ton and rimming actionwas effected for 3 minutes. Thereafter, 20 kg. of Fe-Cr was added and 1minute later, Al was added in a similar way as in example 1. Two bars ofaluminum were inserted into the center of the unsolidified portion.

Analysis of a cold rolled sheet of 0.8 mm., produced from the aboveingot and the mechanical properties of the sheet, as annealed at 720 C.for 3 hours, are as follows:

Analysis:

C: 0.08%, Si: 0.01%, Mn: 0.25% P: 0.013%,

S: 0.014%, Total N: 0.0023%, Cr: 0.010% in the rimmed layer and 0.051%in the core portion, soluble Al: 0.001% in the rimmed layer and 0.005%in the core portion.

Mechanical properties:

Yield point: 21.1 kg./mm.

Tensile strength: 34.0 kg./mm.,

Elongation: 45.8%

Erichsen value: 10.9 mm.

Hardness: HRB 47.3

For comparison purposes, rimmed steel was produced from the same chargeby the conventional method. After rimming action was effected for 25minutes, a small lid was placed on the ingot mold.

Analysis of a cold-rolled sheet of 0.8 mm. thickness produced from theabove comparison ingot and mechanical properties of the sheet, asannealed at 720 C. for three hours, are as follows:

Analysis:

C: 0.04%, Si: 0.01%, Mn: 0.23%, P: 0.01%,

S: 0.015%, Cr: 0.010% both in the rimmed and core portions,

Soluble Al: 0.001% both in the rimmed and core portions.

Mechanical properties:

Yield point: 25.0 kg./mm.

Tensile strength: 34.4 kg./mm.

Elongation: 43.9%

Erichsen value: 10.6 mm.

Hardness: HRB 49.0

The microstructure of the cold-rolled sheet of the present invention, asannealed at 720 C. fo r 3 hours, heated for 3 minutes at 850 C which isalmost equal to the enameling firing temperature, and then quenched insalt water is shown in FIG. 6.

The results of tests for determining the practical usability of partsfor gas cooking-pots for rice and gas ranges produced from the sheets ofthe present invention indicated no blister formation in any of theparts.

What is claimed is:

l. A low-carbon steel ingot having an outer rimmed layer and an innercore layer, the ratio between said outer rimmed layer to the ingotthickness being between about 1:27 and 1 :10 and the ratio betweenrimmed layer and core layer being about 3:70-1:20, the thickness of therimmed layer being at least 0.05 mm., said core layer being alloyed withcarbon-attraction inducing means for causing preferential attraction tocarbon in the core layer, said means including at least one metalselected from the group consisting of Cr, Mn, Al, V and Ti in an amountof 0. 1-0.2% for Cr and Mn, 0.15% for Ti and V, and 0.005-0.020% for Al.as soluble Al, and the content of said alloying metal in the core layerof said steel ingot being greater than that in the rimmed layer of theingot be more than 0.05% for Cr and Mn, by more than 0.02% for Ti and V,and by more than 0.003% for Al as soluble Al, the rimmed layercontaining not more than 0.02% C and the core layer containing0.04-O.15% C.

2. Enameling grade steel sheets made from ingots having an outer rimmedlayer containing not more than 0.02% C and an inner core layercontaining from 0.04-0.15% C, the ratio of the thickness of the outerrimmed layer to the sheet thickness being about between 1:27 and 1:10,said core layer being a1- loyed with an austenite inducing agent forcausing the steel with the agent to preferentially transform toaustenite when heated to a given temperature as compared to said outerrimmed layer, said agent including at least one metal selected from thegroup consisting of Cr, Mn, Ti, V and Al in an amount of 0.10.27% for Crand Mn 0.05-0.15% for Ti and V and 0.0050.020% for Al, as soluble Al,and the content of said alloying metal in the core layer of said steelsheet being greater than that in the rimmed layer of said steel sheet bymore than 0.05% for Cr and Mn, by more than 0.02% for Ti and V, and bymore than 0.003% for Al, as soluble Al.

3. In a method of producing a steel ingot which comprises: pouringlow-carbon molten steel into a mold, permitting the portion of themolten steel which is closest to the walls of the mold to solidify underrimming action until a peripheral rimmed portion is formed, theimprovement which comprises introducing into the still liquid coreportion of the steel mass an austenite inducing agent for causing thesteel with the agent to preferentially transform to austenite ascompared to the rimmed layer, said agent including at least one alloyingmetal selected from the group consisting of Cr, Mn, Al, V and Ti, in anamount between 0.1-0.2% for Cr and Mn, between about 0.05-0.15% for Tiand V, and between about 0.005-0.02% for Al, as soluble Al, therebyobtaining a steel ingot having a rimmed layer and a core portion theratio of thickness between said portions being about 3:704:20, saidalloying elements being capable of preventing carbon migration towardthe surface or of g ather ing carbon precipitating in the outer layertoward the core portion upon heating.

* I III

2. Enameling grade steel sheets made from ingots having an outer rimmedlayer containing not more than 0.02% C and an inner core layercontaining from 0.04-0.15% C, the ratio of the thickness of the outerrimmed layer to the sheet thickness being about between 1:27 and 1:10,said core layer being alloyed with an austenite inducing agent forcausing the steel with the agent to preferentially transform toaustenite when heated to a given temperature as compared to said outerrimmed layer, said agent including at least one metal selected from thegroup consisting of Cr, Mn, Ti, V and Al in an amount of 0.1-0.27% forCr and Mn 0.05-0.15% for Ti and V and 0.005-0.020% for Al, as solubleAl, and the content of said alloying metal in the core layer of saidsteel sheet being greater than that in the rimmed layer of said steelsheet by more than 0.05% for Cr and Mn, by more than 0.02% for Ti and V,and by more than 0.003% for Al, as soluble A1.
 3. In a method ofproducing a steel ingot which comprises: pouring low-carbon molten steelinto a mold, permitting the portion of the molten steel which is closestto the walls of the mold to solidify under rimming action until aperipheral rimmed portion is formed, the improvement which comprisesintroducing into the still liquid core portion of the steel mass anaustenite inducing agent for causing the steel with the agent topreferentially transform to austenite as compared to the rimmed layer,said agent including at least one alloying metal selected from the groupconsisting of Cr, Mn, Al, V and Ti, in an amount between 0.1-0.2% for Crand Mn, between about 0.05-0.15% for Ti and V, and between about0.005-0.02% for Al, as soluble Al, thereby obtaining a steel ingothaving a rimmed layer and a core portion the ratio of thickness betweensaid portions being about 3:70-1:20, said alloying elements beingcapable of preventing carbon migration toward the surface or ofgathering carbon precipitating in the outer layer toward the coreportion upon heating.